Rainbow Smelt (Osmerus mordax)

Rainbow smelt are a non-native, invasive species that have expanded their range in North American freshwater lakes over the last nearly 100 years. Although stocked several times in the Great Lakes, it is generally accepted that the Great Lakes population of rainbow smelt resulted from their being stocked into Crystal Lake, Michigan, in 1912 (Van Oosten 1937). Crystal Lake is connected to Lake Michigan via a stream that flows into the Betsie River, which flows into Lake Michigan near Frankfort. It was not until 1923 that smelt were first taken in Lake Michigan. Subsequently, rainbow smelt spread throughout all the Great Lakes and into many inland waters in Ontario, Minnesota, Wisconsin, and other states and provinces (Evans and Loftus 1987, Rooney and Paterson 2009). Rainbow smelt are also present in the Hudson Bay (Frazin et al. 1994) and Mississippi River (Mayden et al. 1987) drainage systems.

Rainbow smelt invade new waters by moving through connected waters and through human transfer. Rainbow smelt move upstream to new waters during spring spawning migrations, and they move to new waters downstream most likely through passive downstream migration of larva (Hrabik and Magnuson 1999). Human transfer of rainbow smelt has been both intentional and unintentional (Evans and Loftus 1987, Hrabik and Magnuson 1999). Some people may intentionally stock rainbow smelt into lakes because of the mistaken perception that they may improve fishing by increasing the forage base for predatory fish or may create a spring fishery for human consumption during their spring spawning runs. Speculatively, fishermen who rinse out containers that held rainbow smelt captured during spawning may unintentionally spread rainbow smelt; fertilized eggs could be washed into new waters. The construction of canals between previously unconnected waters has also facilitated dispersal.

The presence of rainbow smelt may be predicted as a function of limnological, physical, chemical and biological variables. Mercado-Silva et al. (2006) found that the most important variables in explaining rainbow smelt distribution were lake maximum depth, Secchi depth, and lake surface area. They found that smelt presence was predicted for lakes deeper than 12.3 m (40 ft) with a lake surface area ≥ 21 ha (52 acres) and for lakes with a maximum depth between 8.9 and 12.3 m (29 – 40 ft) with a surface area ≥ 102.8 ha (254 acres) or a Secchi depth ≥ 6.1 m (20 ft). Smelt presence was also predicted in lakes with a surface area < 21.2 ha (52.4 acres), if the lake had a maximum depth greater than 20 m (66 ft). Evans and Loftus (1987) examined the frequency of occurrence of 36 fish species in lakes containing rainbow smelt. They found a strong positive correlation between rainbow smelt and white sucker, smallmouth bass, yellow perch, lake trout, lake whitefish, and cisco. Other fish associated with rainbow smelt included rock bass, pumpkinseed, brown bullhead, and burbot. Rainbow smelt are not found in lakes having a surface pH < 6.0 during May – August (Evans and Loftus 1987). In Minnesota, they are present in Lake Superior, Voyagers National Park, Lake of the Woods, Pokegema Lake near Grand Rapids, and in several lakes in the Arrowhead region of the state.

Life History

Rainbow smelt can be anadromous, moving from saltwater to freshwater streams to spawn, or they can live a completely freshwater existence. Spawning is typically initiated shortly after ice-out when water temperatures rise above 4.4 C (40 F) (Becker 1983). Most spawning activity occurs at water temperatures around 10 C (50 F). Spawning can take place in streams or along shorelines. Rainbow smelt are found in streams and rivers only during their spawning migration. Females sized 185 to 224 mm (7.3 – 8.8 in) in length can release from 21,000 – 41,000 eggs each (Becker 1983). Eggs are negatively buoyant. Eggs are adhesive, attaching to substrate such as gravel, sand, submerged vegetation. Eggs are 0.9 to 1.3 mm in diameter and, depending upon water temperature, they hatch in 2 to 3 weeks (Scott and Crossman 1973, Becker 1983). After hatching, fry are about 5 mm (0.2 in) in length, but grow rapidly and may be 20 – 40 mm (0.8 – 1.6 in) long within a few months. Fry are very slender and nearly transparent (Scott and Crossman 1973). Calculated growth at the end of the first year ranges from 66 to 82 mm (2.6 – 3.2 in) (Becker 1983).

Young of the year rainbow smelt are usually found near shore in warm water from 2-6 m (6.5 – 19.7 ft) deep and yearlings are found in cool water at mid-depths in the pelagic zone (Rooney and Paterson 2009). Adult rainbow smelt may undergo diel vertical migrations, but in Lake Michigan they tended to stay in moderately shallow water less than 60 m (196 ft) in depth (Wells 1968). Occasionally in Lake Michigan small numbers of smelt were found at 91 m (298 ft) (Becker 1983). In Lake Superior, most of the age-0 and age-I smelt were taken at a depth less than 18 m (59 ft), and only an occasional fish was taken at depths below 35 m (115 ft) (Dryer 1966). The age-II and older smelt from trawls were nearly evenly distributed between depths of 18 and 71 m (59 – 232 ft); they were rarely captured above 18 m (59 ft), and none were found below 71 m (232 ft) (Becker 1983). During the winter, smelt were found to occur at depths of 14-18 m (46-59 ft) in Lake Lucerne, Wisconsin (Becker 1983). Smelt have a rather low upper incipient lethal temperature of 18 C (64 F) at 11 C (52 F) acclimation temperature (Evans and Loftus 1987).

Impacts of rainbow smelt are well documented and summarized by Rooney and Paterson (2009). Negative impacts of smelt can arise from predation on larva of other species, competition with other species for food resources, changes in zooplankton size structure, cascading trophic changes that lead to greater phytoplankton biomass, biomagnifications of contaminants, and possibly thiamine deficiency in piscivores feeding on smelt. Recent studies suggest that rainbow smelt predation is an important mechanism for negative effects on native fish communities, but that the effects of competition are also observed (Hrabik et al. 1998, Hrabik et al. 2001). Rainbow smelt are known to negatively impact cisco (lake herring), yellow perch, walleye, lake whitefish, and lake trout (Evans and Loftus 1987, Hrabik et al. 1998, Hrabik et al. 2001, Mercado-Silva et al. 2007, Rooney and Paterson 2009). Rainbow smelt were likely the driving force behind lake-wide collapses of cisco in lakes Superior, Michigan, and Huron during the peak of smelt abundance in those lakes in the 1940s to the 1970s.

Rainbow smelt populations may be controlled through the use of rotenone, intentional overfishing, increasing predator abundance or a combination of these approaches. A change to a 45.7 cm (18 in) minimum size limit on walleye followed by extensive stocking of walleye led to significant declines in rainbow smelt biomass and condition in the Fence Lake Chain of Wisconsin and the possible extirpation of rainbow smelt from Lake Crawling Stone, Wisconsin (Krueger and Hrabik 2005).

General Characteristics

Body is slender and cylindrical. Back is silvery pale green and the sides are iridescent purple, blue, and pink. The underside is white. The body has 26-35 gill rakers, a dorsal fin, an anal fin, pectoral fins, pelvic fins, an adipose fin, and a deeply forked tail fin. It has a pointed snout and large black and silver eyes.